Ah - I have photos but as a new user I found I can’t upload attachments, sorry.
It’s a good idea to email to Siemens, I’ll do that.
On the subject of a pin, both the old hook and the new hook have a transverse hole going through the hook, fairly close to the hearing aid. Looking at the new hook, there is a rectangular section blind hole going into the hook from the “hearing aid joint face”, this hole running parallel with the unthreaded circular hole below it and its width is about the same as the unthreaded hole’s diameter. The transverse “pin hole” - if that is what it is - runs through the side walls of the rectangular hole.
I have seen four of this type of hearing aid, all with the same elbow, and all four had the transverse holes but apparently no pin in it. And looking at the elbow from the side, the bottom half of the mating face of the hook is in contact with the hearing aid as you’d expect , but there appears to be about a parallel 0.5 to 1mm gap between the hearing aid and the top half of the mating face of the hook. so it doesn’t look like any lug or anything can be protruding from the hearing aid into that rectangular hole in the hook.
I’ll see what Siemens say, and many thanks again for your help.